Many catalytic processes are carried out in reactors that contain a series of separate catalytic beds. Reactors used in the chemical, petroleum refining and other industries for passing liquids or mixed-phase liquid/vapor mixtures over packed beds of particular solids are employed for a variety of different processes. Typical of such processes in the petroleum refining industry are catalytic dewaxing, hydrotreating, hydrodesulfurisation, hydrofinishing and hydrocracking. In these processes a liquid phase is typically mixed with a gas or vapor phase and the mixture passed over a particulate catalyst maintained in a packed bed in a downflow reactor.
Uniform distribution of liquid/vapor flow to a catalyst bed is an important consideration in assuring efficient utilization of a catalyst. Efficient catalyst use will result in improved yields and product qualities, increased run lengths, and, if desired, increased throughput through the unit. Uniform flow distribution is also a necessary condition to avoid temperature mal-distribution and hot spots in a reactor.
Typically, mixing devices, e.g., as disclosed in U.S. Pat. No. 6,183,702, are located above an associated fluid distribution system; for example, a horizontally disposed distribution plate or tray. Many different types of fluid distribution systems are known. Some are simple and comprise little more than a pierced or slotted plate. The distribution plate collects the fluid (vapor and liquid), uniformly distributes it across the plate and discharges the fluid on to the catalyst bed. Other types of fluid distribution systems contain a number of downcomers, e.g., “bubble cap” or “chimney” assemblies, which may be disposed over one or more openings in the distribution plate. The bubble cap or chimney provides intimate mixing of the vapor and liquid before the mixed phase fluid is distributed across the catalyst bed below. Deficiencies in existing fluid distribution systems require a large number of slots, bubble caps, or chimneys, which increases cost and complexity and creates more possibility of clogging or other types of failure.
A good flow distribution device should meet the following four basic requirements: provide even distribution of feed to a catalyst bed over a range of gas and liquid rates; be tolerant to certain out-of-levelness of the distribution tray; provide good gas-liquid mixing and heat exchange, and require minimum catalyst bed height to reach equilibrated flow distribution. Other considerations are that they require minimum height to conserve reactor space and are easy to maintain.
One of the key considerations in flow distributor design is the discharge pattern of liquid and gas from the device. A standard chimney distributor provides only some point contacts of liquid with the catalyst bed. It takes a finite bed height to adequately wet the catalyst surface and the reactions to occur.
Bubble cap distributors usually give a mix of some liquid draining along the riser wall, some liquid coming out as a spray and a few droplets occasionally dropping from the center of the riser. In general, we have been satisfied with this pattern of gas and liquid coming out of the riser in a bubble cap distributor. Such a pattern can uniformly wet the catalyst surface directly underneath the distributor. The catalyst bed depth needed for a complete equilibrated flow distribution and catalyst wetting may still be several inches, depending on how consistent and how divergent a spray that is achieved through the flow distributor.
A more uniform and consistent spray pattern and more uniform catalyst wetting in a short length of catalyst bed are desired, and the proposed new design achieves this objective. The new design also satisfies the other requirements as stated earlier.
Known risers or downcomers, such as shown, e.g., in FIG. 7, but without the venturi nozzle and sleeve assembly of the invention shown in one embodiment in FIG. 8, have poor distribution patterns. In some case such known risers/chimneys only distribute two liquid streamlines coming out the chimney.